There are several places around that do cater for tourists that wish to buy a vehicle then on-sell later. However, I'm not sure how these compare price wise with just taking your chances and buying/selling through normal channels. Plus I can't recall any of their names..... I suppose they may make it easier with ownership and insurance (which is not compulsory in NZ). Look on TradeMe for starters, that'll give you some idea as to prices. I'd also enquire with some of the rental companies, as for a long term hire they may do you a good deal. Take a look at Autotrader as well. Land Transport can provide all the info on registration, etc.
Seasonally, I'd shoot for end of summer and autumn - (we don't do fall - this is the most stable time, weather-wise. As long as you avoid the school summer holidays (e/o Dec through e/o Jan), you should be OK. There aren't that many of us (4 million), and what we consider crowded may seem like deserted to those from other countries.
There are some good labs in the main centres. Be aware that processing and film may be more expensive than what you are used to. So, bring lots of film! In Wellington, ImageLab and Wellington Photographic Society (WPS) are the two I'd recommend for developing. Note that ImageLab does all the E6 processing, even that for WPS. I don't believe you can rent their Imacon, alhtough you may be able to get time on the one at the university if you can sort out a contact.